1. Field of the Invention
This invention relates to radio receivers and, more particularly, to automatic gain control of amplifiers within radio receivers.
2. Description of the Related Art
Radio receivers typically include a radio frequency (RF) amplifier that may amplify an incoming RF signal received from an antenna. In a wideband radio receiver such as a frequency modulation (FM) receiver, for example, the RF amplifier is typically a wideband, adjustable gain amplifier, since the RF amplifier passes all frequencies in the receiver's frequency range. The RF amplifier output may be mixed with a local oscillator signal to produce an intermediate frequency (IF) signal. In many receivers, the IF signal may be amplified by an adjustable gain amplifier, which has a narrower pass band than the RF amplifier.
One problem with conventional receivers arises when there is a strong undesired signal present. Many receivers use an automatic gain control (AGC) circuit that may attenuate very strong signals. As such, the AGC circuit may also help attenuate the undesired signal. However, since the RF amplifier is a wideband amplifier, both the desired signal and the undesired signal(s) may be present and contributing to the high signal strength. Thus, if the AGC circuit reduces the RF amplifier gain, the desired signal is also attenuated. This type of attenuation scheme may make adjustment of the IF amplifier gain difficult to optimize and may result in less receiver sensitivity. This may be especially problematic when the desired signal is a relatively weak signal.